The PHP driver will query the database server(s) listed to determine the
primary. So long as it can connect to at least one host listed and find a
primary, the connection will succeed. If it cannot make a connection to any
servers listed or cannot find a primary, a
MongoConnectionException will be thrown.

Tip

You should always provide a seed list with more than one member of the
ReplicaSet. For highest availability you should seed with at least one
server from each of your datacenters.

Warning

The host names that you specify in the seed list must
match the host names in the replica set configuration. This is because the
driver only uses the host names in the replica set configuration to create
the hash for its persistent connections.

For example, if an IP address is used in the seed list and the replica set
is configured with host names, the driver will discard any seed list
connection(s) that differ from the canonical host names reported by the
replica set. Effectively, these non-canonical seed list connections will be
recreated on each request, greatly reducing the benefit of using persistent
connections.

Warning

The driver does not support connecting to different
replica sets with the same name. This extends beyond one script so make
sure to have separate names for each of your replica sets. That also means
that you can not do this:

If the primary becomes unavailable, an election will take place and a
secondary will be promoted to the role of primary (unless a majority vote
cannot be established). During this time
(» 20-60 seconds), the
connection will not be able to perform any write operations and attempts to
do so will result in an exception. Connections to secondaries will still be
able to perform reads.

Note:

The default Read Preference
is to only read from the primary. During the election process there is no
primary, and all read will therefore fail.

It is recommended to use
MongoClient::RP_PRIMARY_PREFERRED Read Preference for
applications that require high availability for reads, as reads will only
be executed on the secondaries when there simply isn't a primary
available.

Once a primary is elected, attempting to perform a read or write will allow
the driver to detect the new primary. The driver will make this its primary
database connection and continue operating normally.

The health and state of a secondary is checked every 5 seconds
(configurable with
mongo.ping_interval)
or when the next operation occurs after 5 seconds. It will also recheck
the configuration when the driver has a problem reaching a server.

Replica set failovers are checked every 60 seconds (configurable with
mongo.is_master_interval),
and whenever a write operation fails when using acknowledged writes.

Caution

Secondaries may be behind the primary in operations, so
your application must be able to handle out-of-date data when using
Read Preferences other then MongoClient::RP_PRIMARY.